The proliferation of a few, antigen-reactive lymphocytes into a large population of effector cells is a fundamental property of adaptive immunity. The cell division that fuels this process is driven by signals from antigen, costimulatory and growth factor receptors, and is controlled by the cyclin-dependent kinase (CDK) cascade. Our work during the last two funding periods has focused on the role of cyclin-dependent kinases in T cell differentiation, anergy and tolerance. Our work established critical roles for CDK2 and its inhibitor p27kip1 in controlling the balance between immunity and tolerance. We showed that mice with a germline deletion of CDK2 accept cardiac allografts under conditions that lead to rejection in wild-type recipients, while mice lacking p27kip1 are highly resistant to tolerance induced by costimulatory blockade. Surprisingly, these factors do not operate through regulation of T cell cycle progression. Instead, we found that CDK2 activity promotes T helper differentiation, and that CDK2-deficient Treg exhibit a gain of suppressive activity. In this renewal application, we will explore this exciting new role for the CDK2 pathway in the control of regulatory T cell function, focused mainly by our findings that Foxp3 is phosphorylated and targeted for degradation by CDK2, and that dysregulated CDK2 activity opposes the induction and stability of Foxp3+ Treg. The proposed work will forward our basic understanding of how Foxp3 and regulatory T cell function is regulated, and will also have important therapeutic implications. Small molecule CDK antagonists are currently in phase I clinical trials, and based on our findings, could potentially be used to promote regulatory T cell function and tolerance in autoimmune and organ transplant patients.